No more soup … optimising your tissue mill chemistry
Imagine a typical scenario faced by a tissue machine operator where chemistry has got out of control. Your machine is using a recycled fibre furnish and making some towel grades; but you are finding it very difficult to reach the wet tensile specification. This important end-user property is delivered by a chemical, a cationic polyamide-epichlorohydrin in fact, but you don’t know that, just that it is ‘wet strength resin’. So, if the lab test for wet tensile is low, let’s just add some more, OK?
Well, no. Perhaps when the next shift arrives, there will be foam overflowing the wire pit. Felt dewatering is suddenly quite poor, but we don’t see that, what we see is wet streaky Yankee coating, poor crepe, web breaks and repeated blade changes. And still the wet tensile is poor. Maybe the new shift operator has seen this before, they shut down, clean the felt, drop the backwater, restart on fresh water and less wet strength resin, and for a while things may be better. Or maybe he just turns up the antifoam pump and things can then get even worse!
What really happened here? The problem with chemistry is that we tend to see only end effects, not visual things like when troubleshooting the mechanical operation of the machine. If we understood a little bit of paper mill chemistry, we might speculate that we had some poor waste rich in anionic trash in the furnish. This trash consumed the wet strength resin before it reacted with the fibre, but by increasing the resin pump, we put more un-reacted chemical into the system, filling the felt, creating foam and destabilising the Yankee coating. But we still don’t know for sure. What we have is a chemical soup we need to unravel, not just to save chemical cost, but to improve the machine operation, as every tissue maker knows that any chemical imbalance will eventually end up on his or her Yankee.
Paper fibres are inherently anionic or negatively charged, due to the carboxyl functional groups on the fibre. Many functional chemicals will have the opposite cationic (positive) charge and thus we have an electrostatic attraction between fibres and chemical we can exploit to put the chemical where it needs to be, i.e. on the fibre. Wet strength, some dry strength, starch, coagulants and de-bonder chemicals all work like this. Already we can see the complexity of several charged functional polymers competing for space on the fibre! However, the furnish stream may also have the so-called ‘anionic trash’ comprising pitch, stickies, fines or ash which will quickly eat up the cationic additive before it can react with the fibre. Now the solution to this might be to neutralise the trash with a non-functional cationic polymer, but how to do this in the right place and right amount?
‘By using these means we can control chemical usage, optimise first pass retention and even improve the crepe efficiency.’
The key to good chemistry management therefore is objective measurement, and the techniques most often applied measure the minute electrical charges on furnish and chemicals. For many years, papermakers have known about particle charge detection (PCD) measurement of the soluble or fine particle charge. PCD can be off-line in the wet lab or on-line and can not only measure the residual chemical in solution but also the troublesome anionic trash. A more recent development is Streaming Zeta Potential (SZP), an off-line technique to measure directly the charge on the fibre. The higher this negative charge is, the better it will retain the cationic additive. Both measurement techniques are now available in a number of easy-to-use commercial options.
To exploit these techniques trained technicians would build a map of SZP and PCD charge for all the stock and water streams, noting chemical additions and modifying these to get the best results. They ask the questions:
• Which furnish stream has the highest chemical retention potential?
• How much anionic trash is present?
• How much fixative to neutralise the trash?
• Did my functional additive retain on the fibre?
And so a new chemical control strategy, based on measurement, not guesswork, is built. By using these means we can control chemical usage, optimise first pass retention and even improve the crepe efficiency.
So, back to our opening story. In fact this describes the case in a Middle East tissue mill which ended up with a typical chemical soup. Trained experts patiently mapped this and suggested the appropriate actions to change addition points and control trash and retention. Not only was this client able to reduce their chemical spend by a staggering 40%, they also were able to achieve much improved tissue machine operation with stable quality and fewer web breaks, thus reducing to just weeks the payback time for the initial investment in instrumentation and training. This story is repeated many times for customers who decided to stop guessing and start to measure their chemistry and ‘get out of the soup’.